You cannot use Exchange Web Services to send task requests. Exchange Web Services can return task requests that are created by MicrosoftOfficeOutlook. If a task request has already been sent, a request to update the task will return an error.

The result of an UpdateItem operation on recurring tasks differs from the result of the UpdateItem operation on a single, nonrecurring task. Changes to an occurrence of a recurring task cause one-off tasks to be generated when the following updates are made:

The status property of a regenerating or nonregenerating recurrent task is set to Completed.

The start date or end date of a nonregenerating recurrent task is changed.

For example, if an UpdateItem request sets the Completed value of a recurring task to true, the UpdateItemResponse will include a new Id and ChangeKey that represent a newly created one-off task. The Id that was included in the request is still valid and the recurring task that is represented by that Id has been updated to represent the next occurrence. The ChangeKey that was included in the request is no longer valid because the recurring task has been updated.

You can use the GetItem operation to get the latest ChangeKey for the recurring task.

For nonrecurring tasks or for the last occurrence of a recurring task, the UpdateItem response returns the same Id that was passed to it and returns the associated updated ChangeKey.